Cache apt install in GitHub Actions.

Essentially, this bundles all installed files into a single zstd compressed tarball cached using actions/cache for future runs. This is much more speedy than going through the apt install flow, but has various caveats (such as post install scripts are not run).

- name: Dependencies
  uses: daaku/gh-action-apt-install@v4
  with:
    packages: luajit libluajit-5.1-dev

Optionally a version can be specified to manually bump the cache key.